Modern drilling techniques employ an increasing number of sensors in downhole tools to determine downhole conditions and parameters such as pressure, spatial orientation, temperature, gamma ray count etc. that are encountered during drilling. These sensors are usually employed in a process called ‘measurement while drilling’ (MWD). The data from such sensors are either transferred to a telemetry device, and thence up-hole to the surface, or are recorded in a memory device by ‘logging’.
The oil and gas industry presently uses a wire (Wireline), pressure pulses (Mud Pulse—MP) or electromagnetic (EM) signals to telemeter all or part of this information to the surface in an effort to achieve near real-time data.
There is a need to control certain mechanical devices such as valves or actuators in many drilling applications and these usually employ electric motors. In such situations, the motor is required to run in a pressure-compensated housing in order to offset large external pressures (usually up to 20,000 psi). In the drilling environment these motors are generally one of two types—brushless or brushed. Both have their advantages and disadvantages—for instance brushed motors do not require sophisticated control circuits and are relatively efficient, and brushless motors have finer positional and rotational control. It is important to note that volume constraints are particularly severe in this environment, so electric motors that make optimum use of their armature coils are normally of the 3-phase variety.
A major issue to be overcome when utilizing most electric downhole motors is that they usually need to move a shaft or lever that is within the external high-pressure environment. In most cases this implies that a high-pressure seal is necessary in order to protect the motor and its associated control electronics at low pressure from ingress by the drilling fluid (‘mud’). Thus the seal must withstand a pressure differential of up to 20,000 psi, often at temperatures of 150° C. to 175° C. This is known to be a point of failure and can absorb significant energy in the form of friction to ensure that the seal is robust enough to withstand the differential pressure. A common method of minimizing this problem is to immerse the motor in an oil bath and communicate the external pressure of the mud to the internal oil via a deformable membrane, such as a rubber sheath. This has the effect of reducing the pressure across the seal to a few psi, thereby requiring a less robust seal that will absorb much less energy from the power source running the motor. The pertinent design issues now involve utilizing an electric motor that can run well while being completely immersed in oil. It is for this reason that most downhole designs make use of brushless motors because they avoid the issue that brushed motors must operate with their commutators and associated brushes in continuous contact. The essential problem is that the commutator is usually rotating at between 2,000 to 6,000 revolutions per minute and at this speed the oil is dragged around by both the armature and the commutator, the latter tending to lift the brushes away as the entrained oil is dragged between them—the ‘hydroplaning’ effect. As soon as the brushes lose contact with the armature the current to the motor stops and power—and control—is lost. A brushless motor has advantages in this respect.
In MP telemetry applications there is a class of devices that communicate by a rotary valve mechanism that periodically produces encoded downhole pressure pulses on the order of 200 psi. These pulses are detected at the surface and are decoded in order to present the driller with MWD information in order to steer the well. These rotary valves are preferentially driven by electric gearmotors, and as the forgoing implies, they will usually be electric and brushless. Because the motors are invariably powered by primary cell batteries it is important that they are efficient. Under conventional circumstances, such as surface applications at atmospheric pressure and with no particularly onerous packaging constraints, the requirements of reliable motor control, motor efficiency and output shaft positional accuracy (in order to set the valve appropriately) are not particularly challenging. But when the downhole motor is brushless and immersed in an oil bath subject to high pressure the need for positional accuracy generally leads to a loss of efficiency, as will be explained as follows.
To achieve the optimum motor torque-speed curve in small motor downhole applications normally requires the motor speed to be typically at least 2,000 rpm. The final valve output mechanism will usually increase and decrease pressure in the mud at a rate of 0.5 to 2 bits per second. This implies that the motor must be geared down in order to match these rates, and also to generate the necessary torque applied to the valve itself so that adequately large pressure pulses can be developed. The valve mechanism in most cases needs the motor to stop and start at specific output positions so that the pressure increase and decrease is well defined according to the prevailing telemetry protocol. Thus the final mechanical valve positional outputs must be monitored, and this information communicated to the motor controller. In a brushless geared-down electric motor as described the necessary output shaft position is normally achieved by some sort of sensor, typically an encoding optical disc; the motor speed and control is by a microprocessor circuit. Both of these means utilize semiconductor components. Problematically, the semiconductors (transistors, diodes, integrated circuits etc.) must be isolated from high pressure or else they will collapse and fail. In situations where pressure must be tolerated the solution for a brushless motor is that one of the armature coils (typically one of three) is used as a sensor to determine speed and position instead of it being used to power the output shaft. This has the effect of significantly reducing the efficiency of a brushless motor. Further, a relatively complicated electronic control circuit housed in a low-pressure environment must be employed.
In summary:                the downhole valve rotary mechanism in most cases requires a rotary output shaft        this implies the beneficial use of a geared-down electric motor        in order to reduce the friction generated by the high differential pressure across the seal separating the external drilling fluid from the internal mechanisms a pressure-compensated housing is employed        the fluid utilized to resist the external pressure is typically oil        the electric motor running in the oil (of finite viscosity) will not suffer brush problems if the motor is brushless        this implies the brushless motor's control and position circuits must be isolated from high pressure        the present state of the art means of achieving brushless motor control and accurate output position employs one of the motor's armature coils        this loss of typically ⅓ of the power-producing coils leads to a serious loss of system efficiency        
It is generally well known that if a brushed motor has to be used the brush lift can be reduced to some extent by some or all of the following means:                reduce the motor's rotational speed        use oil of a lower viscosity        increase the spring force pushing the brushes into the commutator        modify the brush by inserting slots in its bearing surface adjacent to the commutator        
These conventional methods have only limited success, particularly if each parameter has been increased to its practical limit. There have been some attempts to shield the brushes by judicious use of fixed plates (see Grossman, M. I. et al., Elektromashinostroenie i Elektrooborudovanie, no. 25, 1977, p. 107-110), but this type of technique adds significant mechanical complexity and cost. In the downhole industry, present knowledge constrains downhole tool designers to utilize brushless motors in almost all downhole applications.